Scanning of the laser beam is necessary in case of repeatingly processing a tape-shaped work by laser beam, where work is running in a certain direction and that is repeatingly processed substantially parallel to a direction which is not the running direction.
As a method for scanning the laser beam in a high velocity against a work, a method by galvanomirror or polygon mirror is known. FIG. 1 and FIG. 2 show the cases of scanning by the conventional galvanomirror or polygon mirror.
In FIG. 1, numeral 1 is a laser oscillator, numeral 2 is laser beam, numeral 3 is a galanomirror, numeral 4 is a work which runs in a direction of numeral 5. By the galvanomirror 3, the laser beam 2 is scanned as a scanning line 6. As the galvanomirror makes the reciprocating motion, the laser beam is scanned in the directions of the scanning directions 12a and 12b. As the work 4 runs in the direction of the numeral 5, on the work, the processing line is formed zigzag like numeral 7.
Accordingly, in order to obtain a processing condition in which the processing lines 7 are substantially parallel, it is suitable to make the running velocity of the work slow, or inversely, to make the scanning velocity of the laser beam fast; but the former is inferior in efficiency of the processing, and the latter has a limit due to the processing ability by the laser beam.
Similarly, FIG. 2 shows the case that the laser beam is scanned by a polygon mirror 13. The scanning line of this case is numeral 6, and the scanning direction is one given direction of the numeral 12c. As a result, the processing lines 7 become parallel, but it has a demerit that the laser beam 2 cannot be used due to the power reduction when a part of the laser beam 2 is reflected by the edge 14 of the polygon mirror Generally, in case of adopting the conventional scanning method of using a galvanomirror or polygon mirror, there is a problem that only about 50% of time can effectively be used for scanning.
Furthermore, in the case of applying, physical processing such as removing processing or heat treatment or chemical processing for making chemical change by scanning the laser beam against a flat work it is a conventional method to dispose the work vertically against the laser beam at the center of the scanning.
FIG. 3 is a perspective view showing a conventional scanning method. As shown in the figure, since a work 4 is disposed vertically against a laser beam 2a at the center of the scanning, an angle .theta..sub.0 which is made by the laser beam 2a and the work 4 is 90.degree.. When the laser beam is scanned along the scanning line 6, the angle .theta. made by the laser beam 2b and the work 4 is changed, and the angle .theta. is reduced as departing from the center of the scanning.
In the above-mentioned constitution, there are problems as set forth in the following:
(1) As the angle .theta. made by the laser beam 2b and the work 4 varies at respective points on the scanning line 6, the shape of the laser beam spot largely varies. Namely, the shapes of the spot are circular at the center of the scanning and ellipse at a point apart from the center of the scanning, and the ellipticity of the shape increasesas departing more from the center of the scanning, namely as the angle .theta. decreases. Since the variation of the spot shape varies the energy density of the laser beam largely, that, is a large factor to deteriorate the stability of the laser processing. Besides, because of the above-mentioned reason, it is not possible to make the length of the scanning line 6 very large.
(1) When the angle made by the laser beam and the work is around 90.degree., it is likely that such returning light is generated that a part of the laser beam emitted to the work is reflected and passes through the scanning mirror 2 and to the laser oscillator. This returning light becomes remarkable when the work is a material having high reflection coefficient, such as cupper, aluminum or the like. When the returning light returns to the medium of the laser oscillation, the laser beam is amplified or damped depending on its phase, so that the intensity of the laser beam becomes unstable, and it badly influences the processing.